Selective CO oxidation for acetylene converter feed CO control

ABSTRACT

A system and process for acetylene selective hydrogenation of an ethylene rich gas stream. An ethylene rich gas supply comprising at least H 2 S, CO 2 , CO, and acetylene is directed to a first treatment unit for removing H 2 S and optionally CO 2  from the gas stream. A CO oxidation reactor is used to convert CO to CO 2  and form a CO-depleted gas stream. A second treatment unit removes the CO 2  from the CO-depleted gas stream and an acetylene selective hydrogenation treats the CO-depleted gas stream.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of prior copending U.S. application Ser.No. 12/857,709 which was filed on Aug. 17, 2010, the contents of whichare incorporated herein by reference thereto.

FIELD OF THE INVENTION

The invention relates to a system for reducing CO concentration in anethylene rich stream.

DESCRIPTION OF RELATED ART

Industrial processes for producing ethylene include catalytic andthermal cracking of hydrocarbon feedstocks. In at least some cases, thecracking process effluent contains carbon monoxide. For example, certainproduct separation and recovery systems produce a vapor stream rich inethylene and containing hydrogen, methane, acetylene, ethane and othercontaminants such as CO, CO₂, and H₂S that must be removed in order toproduce a high purity ethylene product. Acetylene in polymer gradeethylene is typically limited to a maximum of 5 vol ppm. A typicalpolymer grade ethylene specification is shown in Table 1.

TABLE 1 Typical Polymer Grade Ethylene Specifications Ethylene 99.90 vol% min Methane plus ethane 1000 vol ppm max Ethane 500 vol ppm maxAcetylene 5 vol ppm max C3 and heavier 10 vol ppm max CO 2 vol ppm maxCO₂ 5 vol ppm max Sulfur 2 wt ppm max

Acetylene removal is typically effected by acetylene conversion toethylene via selective hydrogenation. Carbon monoxide (CO) attenuatesthe activity of the commonly used acetylene selective hydrogenationcatalysts and thus excessive CO concentration can be problematic.

Hence it would be beneficial to be able to control the amount of CO thatenters the acetylene conversion unit.

SUMMARY OF THE INVENTION

The present invention relates to controlling CO concentration in astream prior to subjecting the stream to an acetylene selective hydrogencatalyst.

One embodiment of the invention is directed to a system for acetyleneselective hydrogenation of an ethylene rich gas stream comprising: (a)an ethylene rich gas supply comprising at least H₂S, CO₂, CO, andacetylene; (b) a first treatment unit for removing H₂S and, optionally,CO₂ from the gas stream; (c) a CO oxidation reactor to convert CO to CO₂and forming a CO-depleted gas stream; (d) a second treatment unit forremoving the CO₂ from the CO-depleted gas stream; and (e) an acetyleneselective hydrogenation downstream of the CO oxidation reactor.

Another embodiment of the invention is directed to a process foracetylene selective hydrogenation of an ethylene rich gas streamcomprising: (a) supplying an ethylene rich gas comprising at least H₂S,CO₂, CO, and acetylene to a first treatment unit and removing H₂S and,optionally, CO₂ from the gas stream; (b) supplying the H₂S and CO₂ freegas stream to an CO oxidation reactor and converting CO to CO₂ to form aCO-depleted gas stream; (c) supplying the CO-depleted gas stream to asecond treatment unit to remove the CO₂ from the CO-depleted gas stream;and (d) treating the CO-depleted or CO-depleted gas stream to anacetylene selective hydrogenation unit to convert the acetylene toethylene.

These and other embodiments relating to the present invention areapparent from the following Detailed Description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of part of a PetroFCC™ product treating andrecovery system.

FIG. 2 is a schematic of an ethylene rich lean gas preferential COoxidation reactor system in accordance with one embodiment of theinvention.

FIG. 3 is a schematic of ethylene rich lean gas preferential COoxidation reactor system in accordance with another embodiment of theinvention.

The same reference numbers are used to illustrate the same or similarfeatures throughout the drawings. The drawings are to be understood topresent an illustration of the invention and/or principles involved.

DETAILED DESCRIPTION

FIG. 1 is a schematic of a separation scheme for recovering ethylene andpropylene. A vapor stream (2) comprised of ethylene and propylene iscompressed (4) to produce a propylene rich liquid stream (6) and anethylene rich vapor stream (7). The ethylene vapor stream (7) is treatedand concentrated in a primary absorber (8) and a sponge absorber (10) toform an ethylene rich lean sponge gas (12). The lean sponge gas (12)includes other light hydrocarbons, primarily hydrogen, methane,acetylene, ethane and other contaminants such as CO, CO₂, and H₂S thatmust be removed in order to produce a high purity ethylene product.

The ethylene and propylene stream may be obtained from any industrialprocess for producing ethylene including catalytic and thermal crackingof hydrocarbon feedstock product streams. For example, US20080078692discloses a hydrocarbon cracking process and subsequent treatment of theeffluent streams. US 20080078692 discusses various conventional termsand process steps used in processes for recovering ethylene andpropylene after a hydrocarbon cracking process, see especiallyparagraphs 0012-0018, 0034-0041, 0045-0055, and is hereby incorporatedby reference in its entirety,

The ethylene purification scheme shown in FIG. 1 includes an aminetreatment unit (14) to remove H₂S and CO₂ from the ethylene rich leansponge gas (12) forming stream (22). Treatment in the amine treatmentunit reduces the H₂S to less than about 0.1 ppm and CO₂ to less thanabout 50 ppm. The stream (22) is then fed to an acetylene selectivehydrogenation unit (16) to hydrogenate the acetylene into ethylene.

In the scheme shown in FIG. 1, acetylene is hydrogenated upstream of thedemethanizer (18) and ethane-ethylene splitter fractionators (20). Forthis example, stream (22) includes sufficient hydrogen for hydrogenatingthe acetylene in the gas to ethylene. Hence, no additional hydrogen isrequired to be added to the feed stream into the acetylene selectivehydrogenation unit (16). Additionally, the acetylene selectivehydrogenation unit (16) normally operates above ambient temperaturewhile the demethanizer (18) and ethane-ethylene splitter (20) typicallyoperate sub-ambient. Positioning the CO oxidation reactor and acetyleneconversion reactor upstream of the demethanizer lessens feed heating andeffluent cooling duty compared to an arrangement that includes COoxidation and acetylene conversion in the sub-ambient section of theprocess.

The concentration of CO in stream (22) is variable, generally in a rangeof 0 to 6 vol %. It is desirable to maintain the CO concentration of thestream (22) (the acetylene selective hydrogenation reactor feed stream)within a certain operating range, typically about 1 to 0.2 vol %. Ingeneral, as the CO concentration of the acetylene selectivehydrogenation reactor feed stream increases, the operating window of theacetylene selective hydrogenation reactor system and the time betweencatalyst regenerations decreases.

The operating window is the set of operating conditions that enablesselective and stable performance. Specifically, allowing completehydrogenation of acetylene while minimizing hydrogenation of ethylene toethane. The operating window is affected by process conditions includingreactor inlet temperature, feed acetylene, hydrogen, and COconcentrations, space velocity, and catalyst type.

Thus, as discussed above, the feed stream (22) entering the acetyleneselective hydrogenation unit (16) often contains unacceptably highcarbon monoxide (CO) concentrations. The present invention is directedto a process of controlling or reducing the amount of CO in feed stream(22) entering an acetylene selective hydrogenation unit (16).

The feed stream (12) from the sponge absorber contains unacceptably highlevels of CO. An oxidation reactor will oxidize CO in the feed streamusing elemental oxygen as an oxidant: “CO+0.5 O₂→CO₂”. The CO to CO₂conversion selectivity depends on the catalyst choice and composition ofthe feed stream. However, the feed stream (12) from the sponge absorbercontains H₂S which is a catalyst poison for oxidation and must beremoved from the feed stream prior to entering the oxidation reactor.

It was discovered that placing a CO oxidation reactor downstream of theamine treatment unit (14) enables control of the CO concentration in thefeed to the acetylene selective hydrogenation unit (16). As shown inFIG. 2, the ethylene rich stream (12) from the sponge absorber (notshown) flows to a first amine treatment unit (14). For thisillustration, it is assumed that the first amine treatment unit (14)removes both H₂S and CO₂ even though CO₂ removal upstream of theoxidation reactor is not required. Thus, the process does not requireCO₂ removal at this stage.

The ethylene rich stream from the amine treatment unit (14), essentiallyH₂S and CO₂ free, is combined with a stream (32) that provides a sourceof elemental oxygen, for example, air or oxygen enriched air. Thecombined gases A (H₂S and CO₂-depleted stream) flow to the CO oxidationreactor (30). After CO conversion to CO₂, the effluent stream B(CO-depleted stream) continues to a second amine treatment unit (34)downstream of CO oxidation reactor (30). This second amine treatmentunit (34) removes CO₂ from effluent stream B. The CO₂-depleted effluentthen continues to the acetylene selective hydrogenation unit (16).

As also shown in FIG. 2, the amine treating arrangement uses a commonamine regenerator (36) to regenerate rich amine from both the first andsecond amine treatment units (14) and (34). In doing so, amine treatingequipment is minimized. The combination of the preferential CO oxidationreactor (30) and amine treatment unit (14) to remove H₂S enables controlof the CO concentration within a suitable range for subsequent acetyleneconversion via conventional selective hydrogenation technology.

A sensor (38) may be placed in the effluent B stream after the COoxidation reactor (30) to detect the amount of CO in the stream. Thesensor may be placed at any position subsequent to the CO oxidationreactor where CO is present in detectable levels. The sensor may signalwhether the amount of oxygen or air supplied by line (32) should bemodified. The effluent stream B ideally comprises less than about 50ppm-vol CO.

The oxidation temperature in the CO oxidation reactor (30) is typicallybetween about 70° C. and about 160° C.

Suitable catalysts for selectively oxidizing CO using air or oxygenenriched air include, but are not limited to ruthenium metal disposed onan alumina carrier, such as those described in U.S. Pat. No. 6,299,995,hereby incorporated by reference in its entirety. The ruthenium metalcomprises well dispersed ruthenium crystals having an average crystalsize less than or equal to about 40 angstroms. Other suitable catalystsutilize platinum and copper.

Other treatments may be used instead of amine treatment units.Alternative treatment units include absorbers with amine or solvent flowarranged in a cascading relationship. As shown in FIG. 3, an ethylenerich feed gas (12) flows into a first absorber (40) wherein H₂S and CO₂are removed by absorption. The ethylene rich stream from the firstabsorber (40), essentially H₂S and CO₂ free, is combined with a stream(47) that provides a source of elemental oxygen. The combined gases A(H₂S and CO₂-depleted stream) flow to the CO oxidation reactor (42).After CO conversion to CO₂, the effluent stream B (CO-depleted stream)continues to a second absorber (44) downstream of CO oxidation reactor(42). This second absorber (44) removes CO₂ from effluent stream B. TheCO₂-depleted effluent then continues to an acetylene selectivehydrogenation unit (not shown). The CO₂ rich amine from the secondabsorber (44) flows to first absorber (40). The CO₂ and H₂S rich aminefrom the first absorber (40) flows to an amine regenerator (46). Thelean amine from the amine regenerator (46) then flows into the secondabsorber (44). A CO sensor (not shown) may be placed downstream of COoxidation reactor (42) similar to the system shown in FIG. 2 in order tocontrol the amount of air or oxygen added to combined gases A.

In the amine treatment unit (14) shown in FIG. 1, (14) and (34) shown inFIG. 2, (40) and (44) shown in FIG. 3 selective removal of H₂S and CO₂can be achieved using amine-containing chemical solvents. For example,UOP AMINE GUARD™ FS may be used to remove the H₂S and CO₂. Such solventsprovide selective removal of H₂S via amine selection. Other treatmentunits may use other chemical solvents. Chemical solvents are used toremove the acid gases by a reversible chemical reaction of the acidgases with an aqueous solution of various alkanolamines or alkalinesalts in water.

Other treatment units may utilize physical solvents. With a physicalsolvent, the acid gas loading in the solvent is proportional to the acidgas partial pressure. For example, the UOP SELEXOL™ process may be usedwhich uses a physical solvent made of dimethyl ether of polyethyleneglycol. Chemical solvents are generally more suitable than physical orhybrid solvents for applications at lower operating pressures.

As discussed above, in accordance with the present invention, a COoxidation reactor is placed upstream of the acetylene selectivehydrogenation unit to enable control of the CO concentration within asuitable range for the acetylene selective hydrogenation reactionoccurring in the acetylene selective hydrogenation unit. Further aspectsof the invention are therefore directed to a method for controlling theCO concentration in an acetylene selective hydrogenation unit feedstream by preferential CO combustion (i.e. oxidation) with air or oxygenenriched air providing the oxygen.

EXAMPLES

The following examples and tables summarize the expected performance ofthe preferential CO oxidation reactor processing a typical ethylene-richlean gas as shown in FIG. 2. Stream “A” is oxidation reactor feed and“B” is oxidation reactor effluent. The examples assume selectivity basedon a ruthenium on alumina catalyst.

Example 1

The lean gas (i.e. H₂S and CO₂ removed) from the amine treatment unit ismixed with air. The oxygen available for oxidizing CO is controlled tolimit the CO conversion to ˜50%. As shown in Table 2, the COconcentration is reduced from ˜2600 ppm to ˜1300 ppm.

Specifically, stream A is introduced into a CO oxidation reactor andstream B exits the reactor under the following conditions:

Inlet Outlet Reactor Temperature (° F.) 194 221 Reactor Pressure (psia)246.7 Air to Reactor (lbmol/hr) 66

TABLE 2 Stream “A” Stream “B” Mole Mole Fraction Mole % Fraction Mole %H₂O 0.003869 0.387 H2O 0.005189 0.519 Oxygen 0.001314 0.131 Oxygen0.000000 0.000 Nitrogen 0.068311 6.831 Nitrogen 0.068401 6.840 Hydrogen0.106621 10.662 Hydrogen 0.105446 10.545 CO 0.002615 0.262 CO 0.0013030.130 CO₂ 0.000005 0.001 CO₂ 0.001321 0.132 Methane 0.248449 24.845Methane 0.248775 24.878 Acetylene 0.000503 0.050 Acetylene 0.0005040.050 Ethylene 0.485833 48.583 Ethylene 0.486472 48.647 Ethane 0.0764467.645 Ethane 0.076546 7.655 Propylene 0.006035 0.604 Propylene 0.0060430.604

Example 2

The lean gas (i.e. H₂S and CO₂ removed) from the amine treatment unit ismixed with air. The oxygen available for oxidizing CO is controlled tolimit the CO conversion to ˜75%. The CO concentration is reduced from˜2600 ppm to ˜600 ppm, see Table 3. Undesirable side reactions include“H₂+0.5 O₂→H₂O”, as well potential oxidation of light hydrocarbonsincluding olefin products. Assuming sufficient reactant, the COoxidation reactor essentially completely removes CO.

Stream A is introduced into a CO oxidation reactor and stream B exitsthe reactor under the following conditions:

Inlet Outlet Reactor Temperature (° F.) 194 234 Reactor Pressure (psia)246.7 Air to PreFOX Reactor (lbmol/hr) 99

TABLE 3 Stream “A” Stream “B” Mole Mole Fraction Mole % Fraction Mole %H₂O 0.003857 0.386 H₂O 0.005833 0.583 Oxygen 0.001965 0.196 Oxygen0.000000 0.000 Nitrogen 0.070561 7.056 Nitrogen 0.070700 7.070 Hydrogen0.106289 10.629 Hydrogen 0.104530 10.453 CO 0.002607 0.261 CO 0.0006440.064 CO₂ 0.000005 0.001 CO₂ 0.001973 0.197 Methane 0.247674 24.767Methane 0.248161 24.816 Acetylene 0.000501 0.050 Acetylene 0.0005020.050 Ethylene 0.484318 48.432 Ethylene 0.485271 48.527 Ethane 0.0762077.621 Ethane 0.076357 7.636 Propylene 0.006016 0.602 Propylene 0.0060280.603

In view of the present disclosure, it will be appreciated that otheradvantageous results may be obtained. Those having skill in the art,with the knowledge gained from the present disclosure, will recognizethat various changes can be made in the above apparatuses and methodswithout departing from the scope of the present disclosure. Mechanismsused to explain theoretical or observed phenomena or results, shall beinterpreted as illustrative only and not limiting in any way the scopeof the appended claims.

The invention claimed is:
 1. A system for acetylene selectivehydrogenation of an ethylene rich gas stream comprising: (a) an ethylenerich gas supply to supply an ethylene rich feed gas stream comprising atleast H₂S, CO₂, CO, and acetylene; (b) a first treatment unit forremoving H₂S from the feed gas stream forming an H₂S-depleted gasstream; (c) a CO oxidation reactor for converting CO in the H₂S-depletedgas stream to CO₂ forming a CO-depleted gas stream; (d) a secondtreatment unit for removing the CO₂ from the CO-depleted gas stream; and(e) an acetylene selective hydrogenation unit downstream of the COoxidation reactor.
 2. The system of claim 1 further comprising an oxygenor air supply to supply oxygen or air to the H₂S depleted gas streamprior to the CO oxidation reactor.
 3. The system of claim 2 furthercomprising a sensor positioned in the CO-depleted gas stream downstreamof the CO oxidation reactor to detect an amount of CO in the CO-depletedgas stream, wherein the sensor is connected to the oxygen or air supplyto control the supply of oxygen or air to the H₂S and depleted gasstream.
 4. The system of claim 1 wherein the CO oxidation reactorcomprises a catalyst.
 5. The system of claim 4 wherein the catalystcomprises an active material selected from the group consisting ofruthenium, copper, and platinum on a substrate selected from the groupconsisting of alumina, titania, and silica.
 6. The system of claim 5wherein the catalyst is ruthenium on alumina.
 7. The system of claim 1wherein at least one of the first treatment unit and second treatmentunit utilizes a solvent.
 8. The system of claim 7 further comprising aregenerator to regenerate the solvent.
 9. The system of claim 8 whereinthe regenerator regenerates the solvent of both the first and secondtreatment units.
 10. The system of claim 7 further comprising aregenerator to regenerate a flow of CO₂ and H₂S rich solvent stream fromthe first treatment unit, wherein the regenerated solvent stream flowsto the second treatment unit.